1. Field of the Invention
The present invention relates generally to a friction-roller-type continuously variable transmission for automobiles or the like, and more particularly to a speed ratio control device of such transmission.
2. Description of the Related Art
A typical example of the friction-roller-type continuously variable transmission is known, e.g., from JP-A-4-29659. The transmission as known from this publication comprises friction rollers which are arranged in a toroidal groove formed between input and output disks and rotatably supported by rotatable and axially movable roller support members having respective rotational axes which are perpendicular to the common axis of the input and output disks. The speed ratio of the transmission between the input and output disks is determined in accordance with the axial position of the friction rollers. Thus, hydraulic cylinder units are provided to axially move the roller support members and hence friction rollers relative to the input and output disks. For controlling the speed ratio of the transmission, there is provided a speed ratio control device including a step motor which is actuated in accordance with an instructed speed ratio. A lock-up inhibitor valve has a valve spool which is movable in the axial direction as the step motor is actuated. A speed ratio control valve has a valve spool which is provided within a valve sleeve. The control valve sleeve has one end which is connected to one end of the lock-up inhibitor valve spool so that the control valve sleeve is movable in the axial direction together with the lock-up inhibitor valve spool. The other end of the lock-up inhibitor valve spool is provided with a low switch, while the other end of the control valve sleeve can be displaced into contact with a stopper. In this instance, the control valve sleeve is connected to the lock-up inhibitor valve spool via a spring so that the lock-up inhibitor valve spool is capable of performing a stroke motion relative to the control valve sleeve. The low switch is located at a particular position of the lock-up inhibitor valve spool which has not performed a stroke motion relative to the control valve sleeve which, in turn, has displaced into contact with the stopper so as to achieve the lowest speed ratio. The low switch is adjusted so that it is operated at such a particular position of the lock-up inhibitor valve spool.
The above-mentioned continuously variable transmission proved to achieve a satisfactory control of the speed ratio. However, there may be instances wherein it would be desirable to provide the same function with a further simplified construction. For example, the known transmission requires an adjustment of the stopper so that the lowest speed ratio is achieved when the control sleeve is displaced into contact with the stopper, and an adjustment of the low switch so that it is operated at the particular position of the lock-up inhibitor valve spool. Thus, depending upon arrangement of the stopper and the low switch, the adjustments at the two positions may be troublesome and time consuming. Also, it may be difficult to completely accommodate the low switch within the transmission casing. When the low switch partly protrudes from the transmission casing, it may interfere with adjacent components and the position of the low switch may be dislocated in normal use condition of the transmission.